Taeniid cestode infections in man and animals constitute a serious zoonotic and economic problem throughout the world. Fundamental to the continuity of the life cycles of these parasites is the ability of the larval stages to survive for extended periods of time within the tissues of mammalian intermediate hosts. That they are able to do so in immunologically competent animals, which may show a marked and specific antibody-mediated immunity to superinfection, is a paradox challenging to our understanding of resistance mechanisms in infectious disease. Existing hypotheses seek to account for this failure of immunological rejection in terms of isolation of parasites by tissue reactions, molecular mimicry (production of host-like antigens by the parasite) or masking of parasite antigenic determinants by the adsorption of host components. The purpose of this project is to examine immunological events during taeniid metacestode infection. Particular emphasis will be placed on (a) antigen-antibody reactions and their effects upon histopathological and physiological parameters in vivo, and (b) the origin and role of host or host-like components in successful parasitism. The findings will be related to current hypotheses, and to the possibility that survival of tapeworm larvae in tissues may be facilitated rather than impeded by the immune response.